pre-authentication attacks

Ben H bhendin at
Wed May 14 15:17:55 EDT 2014

I was reading up a bit on the history of pre-authentication after hearing a
speaker I generally put all faith into mention something about pre-auth
which I didn't think was accurate (namely that's its use was to help
determine available encryption types...something which I can find no
evidence of).

In any event, my understanding  is that pre-auth is used to prevent an
entity from requesting a TGT without credentials and therefore not being
able to brute force the encryption.

However, there are tools out there which are able to also perform
brute-force attacks against the pre-auth timestamp.  In order to do this
however, it would require the ability to listen on the wire between a
client and a KDC.  Something that may be trivial in certain circumstances
(compromising a single application box could provide a sniff of all users
authenticating to the KDC).

That being said, assuming that all traffic to the KDC is encrypted,
pre-authentication would seem to be superior as I can't request a ticket
without credentials from an insecure location.  If however, we assume that
all traffic between a client and a KDC may be compromised, is
pre-authentication superior?

We don't even need to make repeated attempts for a pre-auth required, we
simply need to listen on the wire for when user's authenticate.
Isn't a known entity like a UTC timestamp eaiser to brute force against
than the encrypted TGT?

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